A wide variety of implantable medical devices (“IMD”) that deliver therapy to or monitor a physiologic condition of a patient have been clinically implanted or proposed for clinical implantation in patients. Such devices may deliver therapy or monitor the heart, muscle, nerve, the brain, the stomach or other organs or tissues. In some cases, IMD's deliver electrical stimulation therapy and/or monitor physiological signals via one or more electrodes or sensor elements, at least some of which may be included as part of one or more elongated implantable medical leads. Implantable medical leads may be configured to allow electrodes or sensors to be positioned at desired locations for delivery of stimulation or sensing electrical activity or other physiological parameters. For example, electrodes or sensors may be located at a distal portion of the lead. A proximal portion of the lead is coupled to an IMD housing, which contains electronic circuitry such as stimulation generation and/or sensing circuitry. In some cases, electrodes or sensors are positioned on an IMD housing as an alternative or in addition to electrodes or sensors deployed on one or more leads.
One example IMD is an electrical stimulation device directed to nerve tissue stimulation, which is sometimes referred to as an implantable nerve stimulator or implantable neurostimulator (“INS”). One particular application of nerve tissue stimulation is vagal nerve stimulation. Vagal nerve stimulation may provide therapeutic effects for heart failure, as well as other conditions including, e.g., depression, epilepsy and various digestion conditions. It is desirable to develop new techniques to control electrical stimulation of nerve tissue.